1. Field of the Invention
The present invention relates to a dielectric ceramic material composition having a perovskite crystal structure. More particularly, the present invention relates to a composition which has a relatively large permittivity, a low dielectric loss and a small temperature coefficient of the resonance frequency.
2. Description of the Prior Art
Dielectric material are widely used in the microwave regions in various processes in a car telephone or personal wireless installation which has recently been put into practical use, a microwave circuit which is now integrated and a gun oscillator. These microwave dielectric material are mainly used for resonators, and they are required to have the following three characteristics.
(1) Since the wavelength is shortened to 1/.sqroot..epsilon. (.epsilon.=dielectric constant) in a dielectric, the dielectric constant should be sufficiently large to comply with the requirement of reduction of the size.
(2) The dielectric loss at a high frequency should be small.
(3) The change of the resonance frequency by the temperature should be small. That is, the temperature dependency of the dielectric constant should be small and the dielectric constant should be stable.
As the dielectric material of this type, there are known a material of the BaO-TiO.sub.2 type, a material of the ReO-BaO-TiO.sub.2 type and a material of the (BaSrCa)(ZrTi)O.sub.3 type.
In the material of the BaO-TiO.sub.2 type, the dielectric constant .epsilon.r is high and 38 to 40, and though the dielectric loss tan .delta. is small and less than 2.00.times.10.sup.-4, a product in which the temperature coefficient .tau.f of the resonance frequency is 0 can hardly be obtained in the single phase. Furthermore, changes of the dielectric constant and the temperature dependency of the permittivity by the change of the composition are large. Therefore, it is difficult to control the temperature coefficient (.tau.f) of the resonance frequency to a low level stably while maintaining a high permittivity and a low dielectric loss. In the material of the ReO-BaO-TiO.sub.2 type, the dielectric constant .epsilon.r is very high and 40 to 60, and a product in which the temperature coefficient .epsilon.f of the resonance frequency is 0 can be obtained. However, the dielectric loss tan .delta. is very large and exceeds 5.0.times.10.sup.-4. In the material of the (BaSrCa)(ZrTi)O.sub.3 type, the dielectric loss tan .delta. is small and less than 2.00.times.10.sup.-4 and a product in which the temperature coefficient .tau.f of the resonance frequency is 0 can be obtained. However, the dielectric constant .epsilon.r is very small and is 29 to 32. Incidentally, the dielectric loss (tan .delta.) referred to in the instant specification is one determined at 500 to 600 MHz according to the re-extrant resonator method.
As is apparent from the foregoing description, none of the above-mentioned known materials have characteristics fully satisfying all of the above three requirements.